The mechanical force that a single fungal cell or bacterial colony exerts on a plant cell may seem vanishingly small, but it plays a key role in setting up some of the most fundamental symbiotic relationships in biology. In fact, it may not be too much of a stretch to say that plants may have never moved onto land without the ability to respond to the touch of beneficial fungi, according to a new study led by Dr. Jean-Michel Ané, a professor of agronomy at the University of Wisconsin-Madison. "Many people have studied how roots progress through the soil, when fairly strong stimuli are applied to the entire growing root," says Dr. Ané, who just published a review of touch in the interaction between plants and microbes in the August 2014 issue of Current Opinion in Plant Biology. "We are looking at much more localized, tiny stimuli on a single cell that is applied by microbes." Specifically, Dr. Ané, Dr. Dhileepkumar Jayaraman, a postdoctoral researcher in agronomy, and Dr. Simon Gilroy, a professor of botany, studied how such a slight mechanical stimulus starts round one of a symbiotic relationship — that is, a win-win relationship between two organisms. It's known that disease-causing fungi build a structure to break through the plant cell wall, "but there is growing evidence that fungi and also bacteria in symbiotic associations use a mechanical stimulation to indicate their presence," says Dr. Ané. "They are knocking on the door, but not breaking it down." After the fungus announces its arrival, the plant builds a tube in which the fungus can grow. "There is clearly a mutual exchange of signals between the plant and the fungus," says Dr. Ané.
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